The present invention relates generally to exposure apparatuses, and more particularly, to a processing unit that transfers a target, such as a mask, a semiconductor substrate, a liquid crystal display plane, and the like, from a supply section to a process chamber that exposes, in a manufacturing process for a semiconductor substrate, a liquid crystal display substrate, and the like.
In manufacturing a fine semiconductor device, such as a semiconductor memory, a logic circuit, and the like, by using a photolithography technique, a reduction projection exposure apparatus has so far been used that uses a projection optical system to project a circuit pattern to a wafer, and the like, thus transferring the circuit pattern.
A minimum size (resolution) to be transferred by a reduction projection exposure apparatus is in proportion to the wavelength of light used for the exposure, and is in inverse proportion to the numerical aperture (“NA”) of a projection optical system. Accordingly, the shorter the wavelength becomes, the better the resolution becomes. For this reason, along with a requirement for a further fine semiconductor device in recent years, development of a shorter wavelength of exposure light has been promoted. As a result, the wavelength of ultraviolet (“UV”) light to be used has become shorter, as with an extra high-pressure mercury lamp (i-line (a wavelength of about 365 nm)), a KrF excimer laser with a wavelength of about 248 nm, and an ArF excimer laser with a wavelength of about 193 nm.
However, due to a rapid demand for a finer semiconductor device, there is a limit to the lithography technique using the UV light. Therefore, in order to efficiently transfer an extremely fine circuit pattern of less than 0.1 μm, a reduction projection exposure apparatus is under development that uses an Extreme Ultraviolet (“EUV”) light with a wavelength shorter than that of the UV light, e.g., a wavelength of about 10 nm to 15 nm. Such an exposure apparatus is called “an EUV exposure apparatus” hereinafter.
Since exposure light with a short wavelength, like EUV light, is heavily attenuated in the atmosphere, the exposure section (processing section) of an exposure apparatus is housed in a vacuum chamber, and the chamber is put in a vacuum atmosphere to reduce the exposure light attenuation. For such a processing unit, since a plate to be processed is transferred between a chamber having the processing section and a plate supply section in the atmosphere, a load lock chamber is provided. See, for example, Japanese Patent Application, Publication No. 10-092724. The plate to be processed is a mask (reticle) that is an original for exposure, and has a circuit pattern. The mask is classified into a transmission mask that transmits the exposure light, and a reflection mask that reflects the exposure light.
FIGS. 10A and 10B are schematic block diagrams showing a configuration of a conventional processing unit 1000. FIG. 10A is its sectional view, and FIG. 10B its top view. In the processing unit 1000, the inside of the process chamber 1100, which houses the processing section 1200, is made a highly vacuum atmosphere. The process chamber 1100 houses a second transfer unit 1110.
In the atmosphere, there is a clean booth 1300 having a carrier setting part 1310 as a plate supply section, and being provided such that it surrounds a first transfer unit 1320, which is structured so as to have access to the carrier setting part 1310 and the load lock chamber 1200.
The load lock chamber 1200 has a first gate valve 1210 shutting off the carrier setting part 1310 in the atmosphere and a second gate valve 1220 shutting off the process chamber 1100.
A description will now be given of an operation of the processing unit 1000. The first transfer unit 1320 takes a mask out of a carrier set in the carrier setting part 1310, and carries the mask up to the load lock chamber 1200. If the mask is transferred to the load lock chamber 1200 and placed on the setting table, the area on the atmospheric side is shut off by closing the first gate valve 1210, and the atmosphere of the load lock chamber 1200 is replaced.
When the atmosphere of the load lock chamber 1200 has been replaced, the second gate valve 1220 is opened, and the mask is taken out by a second transfer unit 1110 in the process chamber 1100, stored in the process chamber 1100, and transferred to the processing section 1120. The mask now set on a mask chuck 1122 in the processing section 1120 is aligned in accordance with a standard.
When an exposure operation is completed, the mask is removed from the mask chuck 1122 by the second transfer unit 1110, and transferred back to the load lock chamber 1200. When the mask is transferred to the load lock chamber 1200, the second gate valve 1220 is closed, and the atmosphere of the load lock chamber 1200 is replaced.
When the atmosphere of the load lock chamber 1200 has been replaced, the first gate valve 1210 is opened, the mask is taken out by the first transfer unit 1320, and is transferred to the carrier setting part 1310.
However, there is a problem in that when a mask is transferred or when the load lock chamber is changed to a vacuum atmosphere, particles (dust) adhere to the mask. For example, when the atmosphere of the load lock is replaced, particulate matter is blown up by an exhaust treatment, which will adhere to the mask. A mask to which such matter adheres does not transmit or reflect exposure light at a place where the particles adhere, which will cause the problem that high-quality exposure is impossible (an occurrence of defective exposure).
Accordingly, various methods are proposed to prevent the particles from adhering to a mask. For example, a method is proposed to apply a pellicle to a mask that protects a pattern surface from particles where a transferred pattern is formed, but in a processing unit that uses a shorter wavelength, such as EUV light, a pellicle may be inapplicable from the restrictions of its materials and structures used. As mentioned above, if the wavelength of the exposure light becomes very short, it will be inappropriate to use a glass material so far used for visible light and ultraviolet light, since absorption of light by the physical substance becomes very large, and further, because there will be no glass materials used in the wavelength region of the EUV light, the pellicle will not be applied.
Further, for the particles adhering to a mask when the load lock chamber is changed to a vacuum atmosphere, a method is proposed for controlling the particles being blown up, e.g., an attempt of a slow start or lowering the exhaust volume at the start of an exhaust operation (i.e., retarding the exhaust). However, if the exhaust is retarded, it will take a longer time to arrive at the state of a high vacuum atmosphere, thus bringing about throughput degradation, as might be expected.
In addition, a removable cover system is also proposed that covers the pattern surface of a mask and transfers the mask protected from particles, and uncovers it just before the exposure operation. Since adherence of the particles scarcely occurs in exposure under a high vacuum environment, the removable cover system is regarded as being effective in protecting the pattern surface of a mask. However, since the conventional removable cover system brings the cover in contact with the mask pattern side or the outside of the pattern, such contact generates particles. Since the pattern surface of a mask used for an EUV exposure apparatus has a multi-layered film laminated with a molybdenum layer and a silicon layer, and this multi-layered film is relatively soft compared to the base material of the mask (plate), it is apt to generate particles, especially through contact with the cover. Particles generated through contact between the mask and the cover get inside the cover, and are very likely to adhere to the pattern surface. Further, the mask used for an EUV exposure apparatus is transferred, usually with its pattern surface, in a direction of gravity (i.e., down), and the transfer mechanism gets into contact with the pattern surface (outer surface of the cover). Such contact creates a factor for causing a particle to occur.
Thus, a method for setting a mask on a frame and transferring this frame is proposed. This method limits the mask in contacting only the frame, and the cover and the transfer mechanism in contacting the frame. For the mask used for an EUV exposure apparatus, low thermal expansion glass, called Zerodure™, is used as a base material to circumvent a thermal effect. In order to keep flatness of the pattern surface, it is necessary to suction-hold the entire surface of the backside of the mask (i.e., the backside of the pattern surface), considering the strength of the base material. Accordingly, in the method that transfers the frame with the mask set, when the entire surface of the backside of the mask is vacuum attracted, the frame is also vacuum attracted. If the frame is higher than the mask, even though slightly, the mask gets off from the frame, and is adsorbed by the mask chuck. In this case, when the mask is detached from the mask chuck, contact is made between the frame and the pattern surface, causing particles. On the other hand, if the mask is higher than the frame, even though slightly, the mask never detaches itself, but the mask becomes stressed by a strength with which the frame is adsorbed to the chuck, which changes the shape of the mask, thus, influencing the flatness of the pattern surface.